1. Field of the Invention
The present invention relates to an ultrasonic diagnosing apparatus for measuring a velocity of a blood stream within a living body by emitting ultrasonic pulses having a given repetition frequency toward the living body, receiving ultrasonic waves reflected by the living body to derive an echo signal and detecting a Doppler shift contained in the echo signal.
2. Description of the Related Arts
There has been known an ultrasonic diagnosing apparatus for measuring a velocity of a blood stream by utilizing the Doppler effect upon the ultrasonic wave and displaying the blood stream on a color monitor in superimposition upon an ultrasonic image. That is to say, when the ultrasonic wave is transmitted toward the blood stream and the ultrasonic wave is reflected by blood cells flowing within a blood vessel, the ultrasonic wave is subjected to the Doppler shift. Such an apparatus is called a color Doppler apparatus and has been widely used. The color Doppler apparatus is described in, for instance, Japanese Patent Application Laid-open Publication Kokai Sho 58-188433 and U.S. Pat. No. 4,799,490. In principle, in such a color Doppler apparatus, a burst-pulsatory ultrasonic beam is emitted from an ultrasonic vibrating element array with a constant time interval and a time period from an instant at which the ultrasonic beam is emitted toward an object to a timing at which an ultrasonic wave reflected by the object is received by the ultrasonic vibrating element array is detected to measure a position of the object within the space and at the same time a frequency shift of the transmitting burst pulse signal is detected to measure a movement of the object within the space. Initially the color Doppler apparatus was used to diagnose the circulation system of the human being such as a heart, but recently it has been utilized to diagnose other organs such as stomach due to the fact that the color Doppler apparatus can provide a large amount of useful information.
In the known color Doppler apparatus, a velocity of the blood stream is detected by utilizing the Doppler effect, so that it is impossible to detect a velocity component of the blood stream in a direction perpendicular to the ultrasonic beam. Moreover, in order to measure a blood stream having a low velocity, it is necessary to set a special Doppler sequence for emitting an ultrasonic wave pulse having a longer time duration than that of an ultrasonic wave pulse for obtaining a usual B-mode ultrasonic image, and further in order to obtain a Doppler signal having a high quality, it is necessary to repeat the Doppler sequence about ten times and to perform an averaging process. This results in a great decrease in a frame rate as compared with the B-mode ultrasonic image and the spatial resolution is also reduced.
In copending U.S. Patent application Ser. No. 07/953,811, there is proposed a novel ultrasonic diagnosing apparatus which can measure a low velocity of a blood stream flowing in a direction parallel with a plane of an ultrasonic probe at a very high spatial resolution without reducing the frame rate by utilizing the improved synthetic aperture method.
The inventor of the instant application has conducted various experiments for the above mentioned ultrasonic diagnosing apparatus utilizing the improved synthetic aperture method and has found the following problems. Particularly, when a blood stream having a very low velocity is measured, relative movement between an ultrasonic probe and an object under observation due to movement of a patient body, breathing and movement of the ultrasonic probe can not be ignored, and Doppler shifts are produced not only by the movement of blood cells within a blood vessel, but also by ordinary or stationary living tissues surrounding the blood vessel. Therefore, a displayed image is wholly colored in blue or red and it is difficult to detect the blood stream easily and accurately. In order to avoid such a drawback, the patient is required to stop breathing during the Doppler sequence, but it is apparent that such a solution is a substantial burden to the patient.
Moreover, also in the usual ultrasonic diagnosing apparatus in which the synthetic aperture method is not utilized, the above mentioned problem of the relative movement between the ultrasonic probe and the living tissues equally occurs, so that it is difficult to measure the blood stream having a low velocity in an easy and reliable manner. Particularly, in an ultrasonic endoscope in which the ultrasonic probe is arranged within a distal end of an insertion section of the endoscope, it is quite difficult to fix the distal end of the insertion section within the patient, and thus the ultrasonic probe is liable to move, so that a velocity of the blood stream can not be measured reliably.